Not Applicable
Not Applicable
Not Applicable
1. Field of the Invention
The invention relates to liquid light guides for transmitting UVC radiation.
2. Description of the Related Art
UVC light guides have been known for a long time. However, they consist of flexible glass fiber bundles. The individual fibers in these bundles are made of synthetic silica glass (refractive index n1) and are sheathed with silica glass doped with fluorine (refractive index n2), thus achieving a maximum optical angle of aperture 2xcex1 of approx. 25xc2x0 (sin xcex1={square root over (n12+L xe2x88x92n22+L )}). Such light guides made of silica glass fiber bundles are, among others, made by the Schott company. They are very expensive and can cover only a comparatively small solid angle of an incoherent UVC radiation source because of the small angle of aperture.
For more than 20 years a liquid light guide has been available for transmitting UVA radiation, said light guide comprising a Teflon(copyright) FEP tube filled with an aqueous CaCl2 solution (n=1.435), as disclosed in DE PS 24 06 424. It has high transmission and excellent photochemical stability in the UVA range (320-400 nm), but is not suitable for transmitting UVC radiation because of photochemical degradation, for example at xcex=250 nm. By shifting the pH value of the aqueous CaCl2 solution from acidic to alkaline the photochemical stability and thereby the transmission stability can be expanded to comprise the UVB range (280-320 nm), cf. DE OS 195 18 147 or U.S. Pat. No. 5,737,473. But even the stabilized solution degrades at intensive radiation in the UVC range.
The patent application DE OS 40 14 363.5 discloses liquid light guides with other liquids for the stable transmission of radiation in the UVB range, said liquids belonging to the group of aqueous phosphate solutions, for example aqueous solutions made of K3PO4, K2HPO4, KH2PO4, NaH2PO4, and Na2HPO4.
One disadvantage of such solutions is the low allowable optical refractive index compared to CaCl2/H2O solutions due to salt precipitation in cold environments, resulting in an unsatisfactory maximum optical angle of aperture. A further disadvantage of aqueous phosphate solutions is their partial water vapor pressure, which is significantly higher than the one of aqueous CaCl2 solutions, so that a liquid light guide with a phosphate solution in Teflon(copyright) FEP tube develops bubbles already after approx. one year despite the extremely low water vapor permeability of the fluorocarbon polymer, which is detrimental to radiation transmission. The recent marketing of new, highly transparent and extremely low refracting fluoropolymers (Teflon(copyright) AF, Hyflon(copyright) AD) by DuPont and Ausimont made it possible to use totally reflecting internal surfaces for light guides having a refractive index which is considerably lower than the one for Teflon(copyright) FEP. Thus a liquid light guide with phosphate solution has an acceptable maximum angle of aperture 2xcex1 in the range of above 50xc2x0.
The new, xe2x80x9camorphousxe2x80x9d fluoropolymers from DuPont and Ausimont permit a liquid phase internal coating of the Teflon(copyright) tube thus acting as polish for the internal surface of the extruded Teflon(copyright) tube so that the reflectivity of in particular short wavelength UV radiation is improved.
DE OS 40 24 445 and 42 33 087 as well as GP P 2 248 312 disclose liquid light guides using Teflon(copyright) AF as internal sheath surface. DE OS 40 24 445 mentions that fluoride solutions, for example KF in H2O, are suitable for short wavelength UV radiation. However, a fluoride solution is disadvantageous in that the obtainable and practical refractive index of such a solution is rot much higher than n=1.35 to 1.36 and that such a solution partially dissolves the plugs of silica glass used to close both ends of the liquid light guide. The silica glass plugs are used as optical windows, and the attack on the polished glass surface has a detrimental effect on the optical transmission.
In view of the shortcoming of the prior art as hereinabove specified, it is desirable to be able to transmit UVC radiation with a liquid light guide comprising a liquid core and a sheath of plastic and having a larger optical angle of aperture, thus achieving a greater transmission capacity. Such light guides may even be less expensive to manufacture than those of the prior art.
The invention provides novel liquid light guides for transmitting UVC radiation in the spectral range of 220-280 nm. The UVC light guide can be coupled to a non coherent source for UVC radiation, e.g. a medium pressure mercury lamp. The UVC radiation emitted by the light guide can be used, for example, for photochemical curing of adhesives and laquers. Preferably, the light guide is flexible.